[GHC] #11684: Fix tests with Clang

[GHC]

#11684: Fix tests with Clang
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        Reporter:  erikd             |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.2.1
       Component:  Compiler          |              Version:  8.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  Other             |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by erikd):

 Apparently there is also a Clang specific `-Qunused-arguments` that
 disables this warning.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11684#comment:1>
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